Retarders serve as a nearly abrasion free, lasting braking means, especially for commercial vehicles and rail guided vehicles, since without abrasive wear retarders possess, for a long period, the advantage of converting braking energy into heat.
In the case of hydrodynamic retarders, the fluid flow energy of a liquid is used for braking, wherein the physical active principle corresponds to that of a hydrodynamic clutch. In accord with this, the retarder develops a rotor-stator relationship in the power fluid, the said stator being immovably affixed in a retarder housing. Upon activation of the retarder, a quantity of oil, commensurate with the desired braking power, is admitted into the rotor blade space whereby the rotor carries the oil with it. The oil reacts thereupon against the stator, giving rise to a braking action which restrains the rotor shaft.
As a result of the above principle, it is possible that a retarder will produce, at a low rotational speed, only a small braking moment.
Under normal driving conditions, the motor rotational speeds lie at a relatively low level, namely, within that rotational speed which is most favorable to the current use. Upon retarder braking, however, a greater circulation of cooling fluid is required and, to this end, a higher motor rotational speed is both desired and necessary for optimal operation.
On this account, the purpose of the present invention is to make available a method which enables, even at originally low rotational speeds, the achievement of a sufficient desired retarder braking power.